Abstract Down syndrome (DS) is the leading cause of intellectual disability and, among other things, it affects cognition, learning, and adaptive skills. In spite of many significant challenges, most people with DS wish to learn the skills that will enable the greatest independence possible in their adult lives (Wehmeyer & Metzler, 1995). One of the greatest barriers to independence for youth with DS is limited reading skills. Although children with DS can learn to read words, their progress typically stalls at the early grade level when the emphasis on reading comprehension increases. Their reading skills may never transition from ?learning to read to reading to learn? (Chall, 1967). It is well known that DS is associated with a cognitive-linguistic phenotype of strengths and weaknesses. Several aspects of this phenotype are directly relevant to reading development, and thus reading development might be atypical in this population (e.g., strengths in visual memory and vocabulary; weakness in syntax). The proposed study has two aims ? Aim 1 is to determine the key components of reading comprehension in DS, following the Simple View of Reading model. Based on a sample of 70 children with DS ages 10-15, the proposed study will determine whether the two major components of reading comprehension ? word identification and language comprehension ? both contribute independently to reading comprehension. The proposed study will go on to test an Extended Simple View which adds three cognitive measures as potential additional components ? verbal working memory, attentional control, and processing speed. Finally, the proposed study will examine components of word identification (phonological decoding and orthographic processing) and language comprehension (vocabulary and syntax). Aim 2 is to test the hypothesis that poor syntax is a key limiting factor in the difficult transition to reading comprehension for children with DS. For this, 40 of the 70 children with DS from Aim 1 (ages 10-15) will be matched with 40 children with non-DS intellectual disability (ID, ages 10-15; oversampling n = 50 to achieve n = 40 of matched participants) and 40 typically developing children (ages 6-10; oversampling n = 50 to achieve n = 40 of matched participants) on word identification ability. It is hypothesized that reading comprehension, along with language comprehension and syntax, will be especially low in the group with DS and that, among several candidates, syntax will be the strongest mediator of the relation between group and reading comprehension. Thus, the proposed study will provide a descriptive breakdown of component (and subcomponent) skills of reading comprehension in DS and identify the skills (e.g., syntax) that most limit the growth of reading comprehension in DS. Findings from the study will be helpful in identifying targets for reading intervention for students with DS.